The present invention relates to apparatus for applying strapping around an object. More particularly, the invention relates to apparatus for tensioning plastic strapping in a loop around an object and forming a heat-sealed joint in the strap loop for securing it about the object.
The present invention is an improvement of the strapping apparatus disclosed in U.S. Pat. No. 3,944,460, issued to G. A. Karr on Mar. 16, 1976, entitled "COMBINATION STRAPPING TOOL FOR PLASTIC STRAP", and in my copending Application Ser. No. 701,306, filed June 30, 1976 and entitled "APPARATUS AND METHOD FOR APPLYING PLASTIC STRAP", which patent and copending application are both assigned to the assignee of the present invention. Those strapping tools include an electric resistance-type heating element movable in use to and from a heating position disposed between the overlapping strap portions for melting the facing portions thereof.
In order to achieve uniformity of bonding of the overlapping strap portions in successive operations of the strapping tool, it is necessary to maintain the temperature of the heating element at a predetermined optimum temperature. In such prior tools, accurate maintenance of the temperature of the heating element has been hindered by variations in the voltage of the electrical power source, which variations affect the temperature to which the heating element is heated. Such variations in the temperature of the heating element result in nonuniformity of joints formed by the strapping tool and may, in severe cases, result in failure of the joints. Furthermore, it has been found that the resistance of no two heating elements is exactly the same, so that there may be considerable variation in temperature among the heating elements of several strapping tools.